Before we speak on a pulse generator we have to differentiate between a square wave and pulse. The pulse and the square wave differ primarily in their duty cycle. Duty cycle is defined as the ratio of the average value of the pulse over one cycle in the peak value of the pulse.
As the average value and peak value are inversely related to their time duration the duty cycle can be defined in terms of the pulse width and the period or pulse repetition time.
Therefore duty cycle = Pulse width/Period
In a square wave the output voltage will have equal on and off times, such that the duty cycle is 0.5 or 50 %. The duty cycle remains unchanged even if the frequency is changed.
In case of a pulse the duty cycle is not constant, it may vary. Short durations of pulses give a low duty cycle. Short duration of pulse has the advantage that the dissipation of power in the component under test is low.
(a) Pulse Characteristics and Terminology:
The characteristics are explained hereunder:
(i) Raise Time : It is defined as the time required for the pulse to increase from 10% to 90% of its normal amplitude.
(ii) Fall Time : It is defined as the time required for the pulse to decrease from 90% to 10 % of its maximum amplitude
(iii) Over Shoot : An over shoot is defined as excess initial raise of amplitude beyond the correct value. It may be visible as a pip or ringing.
(iv) Droop or Sag : Sag is said to occur when the maximum amplitude of the pulse is not constant but decreases slowly,
(b) Types of Pulse Generators :
There are two types of pulse generators. They are :
Active Pulse Generators
Passive Pulse Generators
The active pulse generators are relaxation oscillators. Multivibrators and blocking oscillators are the relaxation type pulse generators.
The passive pulse generators generate a sine wave in original and suitable wave shaping will be done to get the required wave shape.
(c) Pulse Generator :
Pulse generators usually have their range from I Hz to 10 MHz. A linearly calibrated dial will be provided. There will be provision for variation in the duty cycle. There will be two independent output terminals. The pulse generator can be free running or can also be synchronised with external signals.
(d) Explanation of the Block Diagram of a Pulse Generator :
The block diagram of a pulse generator is shown in figure. The frequency control circuit controls the sum of the two currents from the current sources. It applies control voltages to the base of the current control transistors in the two current generators. There are two current sources, ramp capacitor, schmitt trigger and the current switching circuit in the generating loop.
The current source gives a constant current for changing the capacitor (ramp capacitor). The ratio of these two currents is determined by the setting of the symmetry control. This control latter determines the duty cycle of the output waveform. The capacity of the ramp capacitor is selected by the multiplier switch. The last two controls provide decade switching and vernier control of the frequency of the output.
The upper current source supplying a constant current to the ramp capacitor, charges this capacitor at a constant rate and the ramp voltage increases linearly. When the positive slope of the ramp voltage reaches the upper limit set by the internal circuit components, the schmitt trigger changes states.
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| Block Diagram of a Pulse Generator |
The trigger circuit's output goes negative changing i.e. reversing the conditions of the current control switch and the capacitor starts discharging. The discharge rate is linear, controlled by the lower current source. When the negative ramp reaches a predetermined lower level, the schmitt trigger switches back to its original state. This now provides the positive trigger output that reverses the condition of the current switch again cutting off the lower current source and switching on the upper current source. One cycle of operation is complete now. The entire process is repeated. The schmitt trigger circuit provides a negative pulse at a continuous rate.
The output of the schmitt trigger circuit is passed to the trigger output circuit and to the 50Ω and 600Ω amplifiers. The trigger output circuit differentiates the square wave output from the Schmitt trigger, inverts the resulting pulse and provides a positive triggering pulse. The 60Ω amplifier is provided with an output attenuator to allow a vernier control of the signal output voltage. In addition to its free running mode of operation the generator can be synchronised or locked in to an external signal. This is accomplished by triggering the schmitt trigger circuit by an external synchronization pulse. The power supply is a regulated power supply and supplies all the subsystems of the pulse generator.
Tags:
Signal Generators